1. Field of the Invention
The present invention relates, in general, to an audio signal processing apparatus, and more particularly to a sound effect apparatus which performs sound effect processing of an audio signal by using so-called digital sound processor.
2. Description of the Related Art
Recently, many remarkable technical developments have been made in the field of audio equipment. For example, stereophonic systems have been widely used in audio equipment. Digital systems also have been widely used for processing audio signals. These systems make the reproduced sound very similar to an original sound.
Furthermore, a sound effect processing apparatus capable of producing a specific reproduced sound field suitable to a listener's preference, by processing an audio source signal, such as a music signal, has been in great demand in recent years.
For meeting such a demand, there is a sound effect system using a digital sound processor which is called, for example, a surround system. In this surround system, various signal processes are performed for two channel input audio signals. Thus, by outputting four channel sounds, a sound field which surrounds listeners is formed.
For example, a digital sound processor for sound field control is described in "Digital Sound Processor for Sound Field Control LSI, TC9330N", TOSHIBA REVIEW Vol. 42, No. 12, 1987, by Atsushi Kondo, Kazuhito Nakanishi, Yoshihiro Iwamoto. Also, a digital audio system equipped with a digital sound processor is described in "Digital Audio System", TOSHIBA REVIEW, Vol. 42, No. 12, 1987, by Masahide Nagumo, Akira Sasaki, Yoh Yoshida.
Now, assuming a sound field like a concert hall, the acoustics of the hall is considered as an element in development of such a sound effect system. For producing a sound field filled with presence, various reverberation components are added to a basic sound. However, when such a sound, which assumes a specified sound field, is reproduced in a home, as shown in FIG. 5, composite sounds (shown in FIG. 5(c)) consisting of emitted sounds from a speaker (shown in FIG. 5(a)) and a reverberation sound which is a generated sound reflecting at, for example, a wall in a room (shown in FIG. 5(b)) are actually heard. Also, at the product developing stage, there is a necessity to capture low (fine) sounds so that the sound field is assumed to be on the basis of comparatively diverse sounds. However, generally in a home, sound is often listened to at reduced volume because of various restrictions. Therefore, the following problems occur. As shown in FIG. 6, when the volume is low, a low level component of the reverberation sound, e.g., a reverberation component which has a long delay time, is masked by room noise so that it cannot be heard. As a result, a sound without presence is heard in comparison with the sound field which is assumed at the time of developing the sound. On the contrary, if the sound is reproduced at a high volume relative to that at the time of developing the sound, an over acoustic occurs. As is understood from the Fletcher-Munson contour of well known human aural characteristics, in the case of a low level, it becomes too difficult to hear the treble and bass. As a result, the reproduced sound is deprived of power and clarity. As a technique which corrects such a poor sound, a volume control circuit, or the like, is well-known. However, only one correction for basic sound is not enough.